Method for the manufacture of a correlated electron material device

ABSTRACT

Disclosed is a method for the manufacture of a CEM device comprising forming a thin film of a correlated electron material having a predetermined electrical impedance when the CEM device in its relatively conductive (low impedance) state, wherein the forming of the CEM thin film comprises forming a d- or f-block metal or metal compound doped by a physical or chemical vapour deposition with a predetermined amount of a dopant comprising a back-donating ligand for the metal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No.(15/641,124, Attorney Docket No. 252.P016X2), titled “FABRICATINGCORRELATED ELECTRON MATERIAL (CEM) DEVICES,” filed Jul. 3, 2017, andincorporated herein by reference in its entirety.

The present disclosure is concerned with a method for the manufacture ofa correlated electron material (CEM) device, which method comprisesforming a thin film of a correlated electron material for the devicehaving a predetermined electrical impedance when the CEM device in itsrelatively conductive (low impedance) state.

The present disclosure is also concerned with a CEM device comprising athin film of a correlated electron material of predetermined electricalimpedance when the device is in its relatively conductive (lowimpedance) state.

Electronic switching devices are found in a wide variety of electronicdevice types, such as computers, digital cameras, cellular telephones,tablet devices, personal digital assistants and so forth, where they mayfunction as memory and/or logic devices.

Factors of interest to a designer in considering whether a particularelectronic switching device is suitable for such a function, may includephysical size, storage density, operating voltages, impedance ranges,and/or power consumption. Other factors of interest may include cost ofmanufacture, ease of manufacture, scalability and/or reliability.

There appears to be an ever-increasing drive towards memory and/or logicdevices which can exhibit lower power and/or higher speed. Switchingdevices comprising a correlated electron material are at the forefrontof this drive not just because they can exhibit low power and/or highspeed but also because they are generally reliable and easily andcheaply manufactured.

The present disclosure describes an improved CEM device and methods forits manufacture. The CEM device may, in particular, be a switchingdevice. The CEM switching device may find application as a correlatedelectron random access memory (CERAM) in memory and/or logic deviceswhich may be used with a wide range of electronic circuit types, such asmemory controllers, memory arrays, filter circuits, data converters,optical instruments, phase locked loop circuits, microwave andmillimeter wave transceivers, and so forth.

A CEM switching device can exhibit a rapid conductor-to-insulatortransition as compared to other switching devices because the switchingis brought about by an electron correlation rather than by a solid statestructural phase change or by formation of filaments, as is foundrespectively in phase change memory devices and resistive RAM devices.

The rapid conductor-to-insulator transition of a CEM switching devicemay, in particular, be responsive to a quantum mechanical phenomenon incontrast to the melting/solidification or filament formation foundrespectively in phase change and resistive RAM devices. The quantummechanical transition in a CEM switching device between a relativelyconductive state and a relatively insulative state (or between a firstimpedance state and a second impedance state) may occur in several ways.

In one respect, a quantum mechanical transition of a CEM between arelatively insulative/higher impedance state and a relativelyconductive/lower impedance state may be understood in terms of a Motttransition.

As used herein, references to a Mott transition are references totraditional Mott transitions (which are described in the literature aspurely coulombic) as well as references to Mott-like transitions (inwhich the coulombic interaction is modified by some other electroninteraction, such as a dipole-core charge interaction). Accordingly, areference to a Mott insulator includes a reference to a charge-transfer(Mott) insulator, such as nickel (II) oxide, in which the columbicinteraction or screening is modified by a charge transfer complexthrough hybridisation with the oxygen band.

In accordance with a Mott transition, a material may switch from arelatively insulative/higher impedance state to a relativelyconductive/lower impedance state if a Mott transition condition issatisfied. The Mott criteria may be defined by (n_(c))^(1/3) a≈0.26,wherein n_(c) denotes a concentration of electrons, and wherein “a”denotes the Bohr radius. If a threshold carrier concentration isachieved, such that the Mott criteria is met, the Mott transition isbelieved to occur. Responsive to the Mott transition occurring, thestate of the CEM device changes from a relatively higherresistance/higher capacitance state (e.g., an insulative/higherimpedance state) to a relatively lower resistance/lower capacitancestate (e.g., a conductive/lower impedance state).

In another respect, the Mott transition may be controlled by alocalization of electrons. If carriers, such as electrons, for example,are localized, a strong coulomb interaction between the carriers isbelieved to split the bands of the CEM to bring about a relativelyinsulative (relatively higher impedance) state. If electrons are nolonger localized, a weak coulomb interaction may dominate, which maygive rise to a removal of band splitting, which may, in turn, bringabout a metal (conductive) band (relatively lower impedance state) thatis substantially dissimilar from the relatively higher impedance state.

The switching from a relatively insulative/higher impedance state to arelatively conductive/lower impedance state may bring about a change incapacitance in addition to a change in resistance. For example, a CEMswitch may exhibit a variable resistance together with a property ofvariable capacitance. In other words, impedance characteristics of a CEMswitch may include both resistive and capacitive components. Forexample, in a metal state, a CEM switch may comprise a relatively lowelectric field that may approach zero, and therefore may exhibit asubstantially low capacitance, which may likewise approach zero.

Similarly, in a relatively insulative/higher impedance state, which maybe brought about by a higher density of bound or correlated electrons,an external electric field may be capable of penetrating the CEM and,therefore, the CEM may exhibit higher capacitance based, at least inpart, on additional charges stored within the CEM. Thus, for example, atransition from a relatively insulative/higher impedance state to arelatively conductive/lower impedance state in a CEM switch may resultin changes in both resistance and capacitance.

A switching device formed from a CEM may exhibit switching of impedancestates responsive to a Mott-transition in a majority of the volume ofthe CEM comprising the device. The CEM may, in particular, form a “bulkswitch”. As used herein, the term “bulk switch” refers to at least amajority volume of a CEM switching a device's impedance state, such asin response to a Mott-transition. For example, substantially all CEM ofa device may switch from a relatively insulative/higher impedance stateto a relatively conductive/lower impedance state or from a relativelyconductive/lower impedance state to a relatively insulative/higherimpedance state responsive to a Mott-transition.

In one arrangement, shown in FIG. 1, a CEM switching device may comprisea layer of correlated electron material (a CEM layer) sandwiched betweena conductive substrate and a conductive overlay. In this arrangement,the CEM switching device can act as memory storage element. In otherarrangements, the CEM switching device may comprise either a CEM layerprovided on a conductive substrate or a CEM layer provided with aconductive overlay. In these other arrangements, the device comprisessource and a drain regions providing for a flow of current across thedevice.

Referring now to FIG. 1A, a current density versus voltage profile 100of a CEM switching device is shown which illustrates its switchingbehaviour. Based, at least in part, on a voltage applied to terminals ofa CEM device, for example, during a “write operation,” the CEM devicemay be placed into a relatively low-impedance state or a relativelyhigh-impedance state. For example, application of a voltage V_(set) anda current density J_(set) may bring about a transition of the CEMswitching device to a relatively low-impedance memory state. Conversely,application of a voltage V_(reset) and a current density J_(reset) maybring about a transition of the CEM device to a relativelyhigh-impedance memory state.

As shown in FIG. 1A, reference designator 110 illustrates the voltagerange that may separate V_(set) from V_(reset). Following placement ofthe CEM switching device into a high-impedance state or a low-impedancestate, the particular state of the CEM switching device may be detectedby application of a voltage V_(read) (e.g., during a read operation) anddetection of a current or current density at terminals of the CEMswitching device (e.g., utilizing read window 107).

In accordance with FIG. 1A, if sufficient bias is applied (e.g.,exceeding a band-splitting potential) and the aforementioned Mottcondition is satisfied (e.g., injected electron holes are of apopulation comparable to a population of electrons in a switchingregion, for example), a CEM switching device may switch from arelatively low-impedance state to a relatively high-impedance state, forexample, responsive to a Mott transition. This may correspond to point108 of the voltage versus current density profile of FIG. 1A. At, orsuitably nearby this point, electrons are no longer screened and becomelocalized near the metal ion. This correlation may result in a strongelectron-to-electron interaction potential which may operate to splitthe bands to form a relatively high-impedance material.

If the CEM switching device comprises a relatively high-impedance state,current may be generated by transportation of electron holes.Consequently, if a threshold voltage is applied across terminals of theCEM device, electrons may be injected into a metal-insulator-metal (MIM)diode over the potential barrier of the MIM device. In certain devices,injection of a threshold current of electrons, at a threshold potentialapplied across terminals of a CEM device, may perform a “set” operation,which places the CEM device into a low-impedance state. In alow-impedance state, an increase in electrons may screen incomingelectrons and remove a localization of electrons, which may operate tocollapse the band-splitting potential, thereby giving rise to thelow-impedance state.

The current in a CEM switching device may be controlled by an externallyapplied “compliance” condition, which may be determined at leastpartially on the basis of an applied external current, which may belimited during a write operation, for example, to place the CEM deviceinto a relatively high-impedance state. This externally-appliedcompliance current may, in some devices, also set a condition of acurrent density for a subsequent reset operation to place the CEM deviceinto a relatively high-impedance state. As shown in the particulardevice of FIG. 1A, a current density J_(comp) applied during a writeoperation at point 116 to place the CEM switching device into arelatively low-impedance state, may determine a compliance condition forplacing the CEM device into a high-impedance state in a subsequent writeoperation. As shown in FIG. 1A, the CEM device may be subsequentlyplaced into a high-impedance state by application of a current densityJ_(reset)≥J_(comp) at a voltage V_(reset) at point 108, at whichJ_(comp) is externally applied.

The compliance may, in particular, set a number of electrons in a CEMswitching device which may be “captured” by holes for the Motttransition. In other words, a current applied in a write operation toplace a CEM device into a relatively low-impedance memory state maydetermine a number of holes to be injected to the CEM device forsubsequently transitioning the CEM switching device to a relativelyhigh-impedance memory state.

As pointed out above, a reset condition may occur in response to a Motttransition at point 108. Such a Mott transition may bring about acondition in the CEM switching device in which a concentration ofelectrons n approximately equals, or becomes at least comparable to, aconcentration of electron holes p. This condition may be modeledaccording to expression (1) as follows:

$\begin{matrix}{{{\lambda_{TF}n^{\frac{1}{3}}} = {C\text{∼}0.26}}{n = \left( \frac{C}{\lambda_{TF}} \right)^{3}}} & (1)\end{matrix}$

wherein λ_(TF) corresponds to a Thomas Fermi screening length, and C isa constant.

A current or current density in region 104 of the voltage versus currentdensity profile shown in FIG. 1A, may exist in response to injection ofholes from a voltage signal applied across terminals of the CEMswitching device. Here, injection of holes may meet a Mott transitioncriterion for the low-impedance state to high-impedance state transitionat current I_(MI) as a threshold voltage V_(MI) is applied acrossterminals of a CEM device. This may be modeled according to expression(2) as follows:

$\begin{matrix}{{{I_{MI}\left( V_{MI} \right)} = {\frac{{dQ}\left( V_{MI} \right)}{dt} \approx \frac{Q\left( V_{MI} \right)}{t}}}{{Q\left( V_{MI} \right)} = {{qn}\left( V_{MI} \right)}}} & (2)\end{matrix}$

wherein Q(V_(MI)) corresponds to the charged injected (holes orelectrons) and is a function of an applied voltage. Injection ofelectrons and/or holes to enable a Mott transition may occur betweenbands and in response to threshold voltage V_(MI), and threshold currentI_(MI). By equating electron concentration n with a charge concentrationto bring about a Mott transition by holes injected by I_(MI) inexpression (2) according to expression (1), a dependency of such athreshold voltage V_(MI) on Thomas Fermi screening length λ_(TF) may bemodeled according to expression (3), as follows:

$\begin{matrix}{{{I_{MI}\left( V_{MI} \right)} = {\frac{Q\left( V_{MI} \right)}{t} = {\frac{{qn}\left( V_{MI} \right)}{t} = {\frac{q}{t}\left( \frac{C}{\lambda_{TF}} \right)^{3}}}}}{{J_{reset}\left( V_{MI} \right)} = {{J_{MI}\left( V_{MI} \right)} = {\frac{I_{MI}\left( V_{MI} \right)}{A_{CEM}} = {\frac{q}{A_{CEM}t}\left( \frac{C}{\lambda_{TF}} \right)^{3}}}}}} & (3)\end{matrix}$

wherein A_(CEM) is a cross-sectional area of a CEM switching device; andJ_(reset) (V_(MI)) may represent a current density through the CEMswitching device to be applied to the CEM switching device at athreshold voltage V_(MI), which may place the CEM switching device intoa relatively high-impedance state.

FIG. 1B shows a CEM switching device 150 comprising a CEM layer 160sandwiched between a conductive substrate 170 and a conductive overlay180 and a schematic diagram of an equivalent circuit for the switchingdevice.

As previously mentioned, the CEM switching device may exhibitcharacteristics of both variable resistance and variable capacitance. Inother words, the CEM switching device may be considered as a variableimpedance device in which the impedance depends at least in part onresistance and capacitance characteristics of the device if measuredacross device terminals 190 and 190′. The equivalent circuit for avariable impedance device may comprise a variable resistor 192, such asvariable resistor, in parallel with a variable capacitor 194. Of course,although a variable resistor and variable capacitor are depicted in FIG.1B as comprising discrete components, the variable impedance device,such as that shown, may comprise a substantially homogenous CEM.

TABLE 1 Correlated Electron Switch Truth Table Resistance CapacitanceImpedance R_(high) (V_(applied)) C_(high) (V_(applied)) Z_(high)(V_(applied)) R_(low) (V_(applied)) C_(low) (V_(applied)) ~0 Z_(low)(V_(applied))

Table 1 illustrates an example truth table for an example variableimpedance device, such as the device of FIG. 1A. Table 1 shows that aresistance of a variable impedance device, such as that shown, maytransition between a low-impedance state and a substantially dissimilar,high-impedance state as a function at least partially dependent on avoltage applied across the CEM switching device. The impedance exhibitedat a low-impedance state may, for example, be approximately in the rangeof 10.0-100,000.0 times lower than an impedance exhibited in ahigh-impedance state. However, the impedance exhibited at alow-impedance state may be approximately in the range of 5.0 to 10.0times lower than an impedance exhibited in a high-impedance state. Table1 also shows that a capacitance of a variable impedance device, such asthe device shown, may transition between a lower capacitance state,which may, for example comprise an approximately zero, or very little,capacitance, and a higher capacitance state that is a function, at leastin part, of a voltage applied across the CEM switching device.

The CEM switching device may be placed into a relatively low-impedancememory state, such as by transitioning from a relatively high impedancestate, for example, via injection of a sufficient quantity of electronsto satisfy a Mott transition criteria. In transitioning a CEM switchingdevice to a relatively low-impedance state, if enough electrons areinjected and the potential across the terminals of the CEM deviceovercomes a threshold switching potential (e.g., V_(set)), injectedelectrons may begin to screen. As previously mentioned, screening mayoperate to delocalize double-occupied electrons to collapse theband-splitting potential, thereby bringing about a relativelylow-impedance state.

It has now been surprisingly found that the electrical (hole)conductivity of a thin film of a transition metal compound, such as atransition metal oxide, which is doped by a back-donating ligand, may bedirectly proportional to the concentration of the back-donating ligandin the transition metal compound.

Without wishing to be bound by theory, it is surmised that the backdonation between the transition metal and dopant ligand in thecorrelated electron material predominantly controls the lowering theFermi energy (and energy gap) between normally equivalent “d”- or“f”-orbitals on the metal and for providing the necessary p-typedegeneracy for hole conduction of the correlated electron material inits relatively conductive (low impedance) state.

The electrical impedance of a thin film of a correlated electronmaterial may, therefore, be carefully controlled (and finely tuned) in aphysical or chemical vapour deposition process, such as an atomic layerdeposition, by precisely controlling the incorporation of a dopantcomprising a back-donating ligand through the deposition processparameters.

Accordingly, the present disclosure provides a method for themanufacture of a CEM device comprising forming a thin film of acorrelated electron material having a predetermined electrical impedancewhen the CEM device in its relatively conductive (low impedance) state,wherein the forming of the CEM thin film comprises forming a d- orf-block metal or metal compound doped by a physical or chemical vapourdeposition with a predetermined amount of a dopant comprising aback-donating ligand for the metal.

References herein to a “back-donating” ligand are references to a ligandexhibiting metal-ligand bonding wherein the ligand supplies (donates)one or more electrons (i.e. electron density) to a metal (i.e. metalatom or ion) and at the same time the metal (i.e. atom or ion) supplies(back-donates) electron density from an occupied d- or f-orbital to anunoccupied π*-orbital or a σ*-orbital having a n-symmetry on the ligand.

In embodiments, the method may form the CEM thin film with apredetermined amount of one or more of a dopant comprising aback-donating ligand.

Suitable back-donating ligands include carbonyl (CO), nitrosyl (NO), anisocyanide (RNC, where R is H, C₁-C₆ alkyl or C₆-C₁₀ aryl), dioxygen(O₂), dihydrogen (H₂), an alkene (for example, ethene), an alkyne (forexample, ethyne) or phosphinyl (R₃P where R is C₁-C₆ alkyl or C₆-C₁₀aryl).

In embodiments, the method may form a CEM thin film comprising one ormore of a d-block or f-block metal or metal compound. It may, forexample, form a CEM thin film comprising one or more of a transitionmetal, lanthanide or actinide or a transition metal oxide, for example,one or more of nickel oxide, cobalt oxide, iron oxide, or one or more arare earth oxide, such as yttrium oxide or indium oxide.

In embodiments, the method may form a CEM thin film of general formAB:L_(x) (for example, NiO:CO) wherein the AB denotes, for example, atransition metal compound, such as a transition metal oxide, L_(x)denotes an extrinsic back-donating ligand for the metal and x indicatesthe number of units of ligand for one unit of the transition metal ortransition metal compound. The value of x for any specific ligand andany specific combination of ligand with a transition metal oxide may bedetermined simply by balancing valences.

In embodiments, the method may form a CEM thin film of a transitionmetal oxide, such as NiO:L_(x) wherein the dopant comprising aback-donating ligand comprises a molecule of the formC_(a)H_(b)N_(d)O_(f) (in which a≥1, and b, d and f≥0) such as: carbonyl(CO), cyano (CN⁻), ethylenediamine (C₂H₈N₂), 1,10-phenanthroline(C₁₂H₈N₂), bipyridine (C₁₀H₈N₂), pyridine (C₅H₅N), acetonitrile (CH₃CN)and cyanosulfanides such as thiocyanate (NCS⁻).

In one embodiment, the forming of the CEM thin film layer comprises aphysical vapour deposition, such as reactive sputtering, of a transitionmetal compound, for example, a transition metal oxide, in an atmosphereof a gaseous oxide, such as carbon monoxide (CO).

In other embodiments, the forming of the CEM thin film layer comprises achemical vapour deposition, such as an atomic layer deposition (ALD), ofa transition metal compound, for example, a transition metal oxide.

The atomic layer deposition may form the CEM thin film utilisingseparate precursor molecules AX and BY, according to the expression (4)below:

AX _((gas)) +BY _((gas)) =AB _((solid)) +XY _((gas))   (4)

wherein “A” of expression (4) corresponds to the transition metal, and“AB” a transition metal compound, such as a transition metal oxide.

In these embodiments, “X” of expression (4) may comprise one or more ofa ligand, such as an organic ligand, for example, amidinate (AMD),cyclopentadienyl (Cp), di(ethylcylcopentadienyl) ((EtCp)₂),bis(2,2,6,6-tetramethylheptane-3,5-dionato) ((thd)₂), acetylacetonate(acac), bis(methylcyclopentadienyl) ((MeCp)₂), dimethylglyoximate(dmg)₂, (apo)₂ where apo=2-amino-pent-2-ene-4-onato, (dmamb)₂ wheredmamb=1-dimethylamino-2-methyl-2-butanolate, (dmamp)₂ wheredmamp=1-dimethylamino-2-methyl-2-propanolate andbis(pentamethylcyclopentadienyl) (C₅(CH₃)₅)₂.

Suitable precursor molecules AX include, therefore, organometalliccompounds of the transition metals (especially the late transitionmetals) having one or more of these ligands alone or in combinationtogether with other ligands.

Accordingly, in some embodiments, a nickel based precursor AX (NiX) maycomprise, for example, nickel amidinate (Ni(AMD)),bis(cyclopentadienyl)nickel (Ni(Cp)₂), nickel acetoacetonate(Ni(acac)₂), nickel dimethylglyoximate (Ni(dmg)₂),bis(ethylcyclo-pentadienyl)nickel(Ni(EtCp)₂),bis(methylcyclopentadienyl)nickel (Ni(CH₃C₅H₄)₂), nickel2-amino-pent-2-en-4-anate (Ni(apo)₂), Ni(dmamb)₂ wheredmamb=1-dimethylamino-2-methyl-2-butanolate, Ni(dmamp)₂ wheredmamp=1-dimethylamino-2-methyl-2-propanolate orbis(pentamethyl-cyclopentadienyl)nickel (Ni(C₅(CH₃)₅)₂).

The precursor “BY” in expression (4) may comprise a gaseous oxide (as anoxidant), such as water (H₂O), oxygen (O₂), ozone (O₃), nitric oxide(NO), nitrous oxide (N₂O) hydrogen peroxide (H₂O₂) or plasma-formedoxygen radical (O.).

In one embodiment, the method comprises forming the CEM thin film by anatomic layer deposition of one or more of a transition metal (especiallya late transition metal) carbonyl and an oxidant.

Suitable transition metal carbonyls include, for example, nickeltetracarbonyl (Ni(CO₄), carbonyl pentacarbonyl (Fe(CO)₅), dicobaltoctacarbonyl (Co₂(CO)₈), triruthenium dodecacarbonyl (RU₃ (CO)₁₂),tetrarhodium dodecacarbonyl (Rh₄(CO)₁₂), triosmium dodecacarbonyl(Os₃CO₁₂), tetrairidium dodecacarbonyl (Ir₄(CO)₁₂).

In embodiments, the atomic layer deposition may employ an organometallicprecursor comprising a back-donating or other ligand and a gaseousoxidant, such as oxygen, wherein the partial pressure of the oxidant iscontrolled (by for example, a mass flow controller) so that the thinfilm is first formed with an excess of metal ion.

In this embodiment, the partial pressure of the oxidant may generally beless than that necessary for formation of stoichiometric transitionmetal oxide (which is not doped), and may also be less than thatnecessary for the complete oxidation of interstitial metal ion.

Note that the predetermined electrical impedance of the CEM thin filmwill be inversely proportional to the concentration of the back-donatingligand in the metal compound.

Note also that the electrical conductivity (or conductance) of the CEMthin film will be predetermined and directly proportional to theconcentration of back-donating ligand in the metal compound.

Of course, the control of the predetermined amount of back-donatingligand may also depend on the number of back-donating ligands in thetransition metal precursor AX as well as the deposition processconditions.

The predetermined amount of dopant comprising a back-donating ligand mayvary between amounts providing for an atomic concentration of backdonating ligand between 0.1% and 15.0%, for example, between 0.1% and10.0%. As referred to herein the term atomic concentration relates tothe concentration of atoms in the finished material that derive from thesubstitutional ligand. For example, in the case that the substitutionalligand is carbonyl, the atomic concentration of carbonyl in percentageterms is the total number of carbon and oxygen atoms that are incarbonyl motifs in the finished material divided by the total number ofatoms in the finished material multiplied by 100.

The predetermined electrical impedance of the CEM thin film can bedetermined or selected in advance of forming the CEM thin film for adevice. The selection may, for example, be made by experiment or byconsultation with a library correlating the electrical conductivity(with dopant concentration) of CEM thin films of a transition metalcompound, such as a transition metal oxide, with suitable precursors andatomic layer deposition process conditions.

In embodiments, the method may form a CEM thin film having apredetermined electrical impedance which is selected to match theconductivity of the device to the (mutual) transductance (g_(m)) of oneor more in-series field effect transistors (FETS).

In one embodiment, therefore, the method may form a CEM thin film havinga (predetermined) electrical conductivity between 10³ S/m and 10¹⁰ S/m.

In these and other embodiments, the method may form a CEM thin filmhaving a thickness from 1 nm to 10 nm, for example, 3 nm or 5 nm.

Further, the method may form a CEM thin film of width above 3 nm, forexample 5 nm or 10 nm or more. The method may form a CEM thin film ofcross-sectional area above 25 nm², for example, 500 nm² or more. In oneembodiment, the method may form a thin film having minimumcross-sectional area 25 nm² with electrical conductivity approaching 10⁵S/m or more.

In embodiments, the method may form a thin film of correlated electronmaterial comprising a transition metal oxide having a dopant or dopantscomprising different back-donating metal ligands. The differentback-donating metal ligands may facilitate fine tuning of the electricalimpedance of the CEM thin film.

The atomic layer deposition may provide that the thin film is “born-on”viz., the device is in its conductive state when it is first formed. Itmay also provide that the thin film comprises a transition metal inseveral oxidation states without a substantial presence of free metal.

In certain embodiments, the method may further comprise annealing thedeposited thin film in an atmosphere comprising an oxidant (such asoxygen (O₂) or carbon monoxide (CO)) at a partial pressure andtemperature maintaining the ratio of concentration of back-donatingmetal ligand, such as carbonyl ligand, to concentration of thestoichiometric transition metal compound, such as transition metaloxide.

It has also been found that the magnitude of the (hole) leakage currentof a CEM device in its relatively insulative (high impedance) state maybe directly proportional to the extent of doping by a back-donatingmetal ligand.

In that case, the electrical performance of a CEM device may be betterthe smaller the cross-sectional area of the film.

Accordingly, the method may also comprise forming the CEM thin film witha predetermined cross-sectional area and a predetermined electricalimpedance selected to provide a predetermined (hole) leakage currentdensity for a device in its relatively insulative (high impedance)state.

The method may provide for CEM devices having dimensions which are (10%to 50%) smaller as compared to those of the CEM devices presently in theart.

A CEM device comprising a thin film of a correlated electron materialhaving a predetermined electrical impedance when the CEM device is inits conductive state wherein the thin film comprises a d- or f-blockmetal or metal compound doped with a predetermined amount of aback-donating ligand for the metal.

In embodiments, the CEM thin film may comprise one or more of a d-blockor f-block metal or compounds of a d-block or f-block metal. It may, forexample, comprise one or more of a transition metal oxide, for example,nickel oxide, cobalt oxide, iron oxide and/or one or more a rare earthoxide, such as yttrium oxide or indium oxide.

In one embodiment, the CEM thin film comprises a transition metal(especially a late transition metal) compound, such as a transitionmetal oxide, which is doped with a predetermined amount of a dopantcomprising a carbonyl ligand.

As mentioned above, suitable back-donating ligands comprise not onlycarbonyl (CO) but also nitrosyl (NO), an isocyanide (RNC where R is H,C₁-C₆ alkyl or C₆-C₁₀ aryl), dioxygen (O₂), dihydrogen (H₂), an alkene(for example, ethene), an alkyne (for example, ethyne) and phosphinyl(R₃P where R is C₁-C₆ alkyl or C₆-C₁₀ aryl).

Other suitable back-donating ligands may comprise fluoride (F), chloride(Cl), bromide (Br), sulfur (S), carbon (C) or be selected from moleculesof formula C_(a)H_(b)N_(d)O_(f) (in which a≥1, and b, d and f≥0) suchas: carbonyl (CO), cyano (CN⁻), ethylenediamine (C₂H₈N₂),1,10-phenanthroline (C₁₂H₈N₂), bipyridine (C₁₀H₈N₂), pyridine (C₅H₅N),acetonitrile (CH₃CN) and cyanosulfanides such as thiocyanate (NCS).

As mentioned above, the electrical impedance of the CEM thin film can bedetermined or selected in advance of manufacturing the device. Thedetermination or selection may, for example, be made by experiment or byreference to a library correlating the electrical conductivity (anddopant concentration) of CEM thin films of a transition metal compound,such as a transition metal oxide, with suitable precursors and atomiclayer deposition process conditions.

The predetermined amount of dopant comprising back-donating metal ligandmay vary between amounts providing an atomic concentration of ligandbetween 0.1% and 15.0%, for example, between 0.1% and 10.0%.

In embodiments, the electrical conductivity of the CEM thin film has avalue between 10³ S/m and 10¹⁰ S/m.

In one embodiment, the predetermined electrical impedance of the CEMthin film has a value which is selected to match the conductivity (orconductance) of the device with the (mutual) transductance (g_(m)) ofone or more in-series field effect transistors (FETS) within a selectedcircuit.

In these and other embodiments, the CEM thin film may have a thicknessfrom 1 nm to 10 nm, for example, 3 nm. The CEM thin film may have a(square or rectangular) width between 3 nm and 25 nm, for example 5 nm.The CEM thin film may have a cross-sectional area as low as 25 nm²although any cross-sectional area found in the art is possible. In oneembodiment, the thin film has cross-sectional area 25 nm² and anelectrical conductivity approaching or above 10⁵ S/m or 10⁷ S/m or 10⁸S/m.

In some embodiments, the transition metal compound comprises atransition metal oxide (TMO:dopant) such as nickel oxide (NiO), an ironoxide (for example, FeO), a cobalt oxide (for example, CoO), a rutheniumoxide (for example, RuO₂), rhodium oxide (for example, Rh₂O₃), osmiumtetroxide (OsO₄), iridium oxide (IrO₂), and mixtures thereof.

In a preferred embodiment, however, the transition metal compoundcomprises nickel oxide (NiO:dopant) which is doped with a predeterminedamount of carbonyl ligand.

As mentioned above, the magnitude of the (hole) leakage current of a CEMdevice (in its insulative state) may be directly proportional to theextent of doping by a back-donating metal ligand.

In that case, the performance of the CEM device may be improved byproviding that area of the CEM thin film (and device) is smaller.

In certain embodiments, the device may have a (hole) leakage current(the ubiquitous parasitic MIM diode current) density in its relativelyinsulative (high impedance) state of less than or equal to 500 A/cm²,for example, 250 A/cm² or 100 A/cm² or 50 A/cm².

A method for tuning the electrical conductivity (or impedance) of a CEMdevice in its relatively conductive (low impedance) state to thetransductance of one or more in-series field effect transistors in anelectrical circuit, which method comprises forming a first thin film ofa correlated electron material by a physical or chemical vapourdeposition of a d- or f-block metal or metal compound which is doped bya first amount of back-donating ligand for the metal; comparing theelectrical conductivity of the first thin film in its relativelyconductive state to the transductance of the one or more in-series fieldeffect transistors; and repeating these steps by forming one or moreother thin films of a correlated electron material each having adifferent amount of back-donating ligand for the metal as compared tothe first thin film layer until the electrical conductivity of one ofthe other thin films in its relatively conductive state matches theconductivity of the device to the transductance of the one or morein-series field effect transistors.

Of course, the first and the one or more other (additional) thin filmsof correlated electron material are formed independently of each otherby, for example, an atomic layer deposition.

In these and other embodiments, the method may comprise forming aplurality of CEM devices wherein a first CEM device is associated withthe first thin film and one or more other CEM devices arecorrespondingly associated with the one or more other CEM thin films.The first CEM device and the one or more other CEM devices may beidentical save in the first and one or more other CEM thin films.

The method may, therefore, provide for direct tuning of a CEM devicewithin an electrical circuit.

Of course, the first and different amounts of back-donating metal ligandin the first and additional CEM thin films may be brought about bycontrol of the process conditions, such as temperature, flow rate(partial pressure) of oxidant for the atomic layer deposition, as wellas by use of different oxidants for differently doped additional thinfilms.

The predetermined amount of back-donating metal ligand in each CEM thinfilm may vary between amounts providing an atomic concentration ofcarbon, oxygen, hydrogen or nitrogen between 0.1% and 15.0%, forexample, between 0.1% and 10.0%.

In a fourth aspect, the present disclosure provides a library comprisinga database for storing and retrieving data relating the electricalimpedance (or conductivity) of a CEM thin film comprising a d- orf-block metal or metal compound doped with a dopant comprising aback-donating ligand for the metal to standard apparatus, standardreactants and process parameters for obtaining the CEM layer by chemicalvapour deposition such as an atomic layer deposition.

The methods and device according to the present disclosure will now bedescribed in more detail having regard to the following embodiments andthe accompanying drawings in which:

FIG. 1A shows a schematic illustration of a current density versusvoltage profile of a CEM switching device;

FIG. 1B shows a schematic illustration of the CEM switching device ofFIG. 1A and a schematic diagram of an equivalent circuit for theswitching device;

FIG. 2 is a schematic illustration of apparatus for implementing methodsfor forming the storage element;

FIG. 3 is a flow diagram showing a scheme for forming a CEM switchingdevice of FIG. 1 having a predetermined electrical impedance in itsconductive state; and

FIG. 4 shows pulse profiles for atomic layer deposition which may beused for forming CEM switching devices having different predeterminedelectrical impedance in their conductive states.

Referring now to the accompanying drawings FIG. 2 shows an apparatus 201for forming a thin film by atomic layer deposition (or chemical vapourdeposition). The apparatus comprises a process chamber 202 connected toup line sources of a metal-containing reactant precursor 203 such astetracarbonyl nickel Ni(CO)₄, a purge gas N₂ and several reactantprecursors 204 comprising oxidants of differing reactivity for themetal-containing reactant precursor, O₂, H₂O and NO. The reactivity ofthese reactant precursors has the order O₂>H₂O>NO.

The process chamber 202 includes a platform (not shown) providing forthe placement of a semiconductor substrate in the middle of the processchamber 202 and equipment (not shown) regulating the pressure,temperature and gas flow within the chamber in combination with a vacuumpump 204 connected to downline of the process chamber 202. The vacuumpump 204 evacuates to an abatement chamber 205 where the reactantprecursors and by-products of reaction are made safe before they enterthe environment.

The apparatus includes a plurality of independently operable valveswhich help regulate the gas flow up line and downline of the processchamber. The up-line valves allow the reactant precursors and purge gasto enter the process chamber 202 sequentially and enable a selection ofone or other oxidant or a particular combination of oxidants forreaction with tetracarbonyl nickel Ni(CO)₄ and/or the surface of thesubstrate.

The equipment regulating the gas flow in the pressure chamber includes amass flow controller 206 providing very precise and highly repeatablecontrol of the amount of oxidant introduced into the process chamber ina predetermined time period.

The apparatus is first prepared for use by loading the platform with thesemiconductor wafer and evacuating the chamber 202 by operating thevacuum pump 204 and opening the up-line valves for the purge gas N₂.During the purging, the process chamber 202 is heated to the temperaturewhich has been selected for the thin film forming process.

Referring now to FIG. 3, the use of a CEM device having desiredelectrical impedance in its conductive state relies upon selection of aCEM layer for the CEM device which has a predetermined electricalconductivity when the device is in its conductive state.

The selection of the CEM layer may be made from a library of CEM layersof known electrical conductivity in a standard CEM switching device(similar to that described above). The library may contain sub-librariescomprising a plurality of CEM transition metal oxide layers in whicheach CEM layer comprises the same transition metal oxide but is doped toan extent which is different as compared to any other CEM layer. Ofcourse, the library may also contain sub-libraries which refer todifferent transition metal oxides and the same or differentback-donating metal ligands.

The library may, for example, refer to a sub-library of CEM layerscomprising nickel oxide wherein each CEM layer is doped by a carbonylligand to an extent which different as compared to any other CEM layer.

The library may relate the electrical conductivity of a CEM layer toapparatus, reactants and parameters for obtaining the CEM layer by anatomic layer deposition. It may, for example, specify apparatusaccording to FIG. 2 as well as reactants and parameters for use with theapparatus.

The library may specify reactants and operating parameters for apparatusthat provide essentially for control of the extent of doping of the CEMlayer by control of the partial pressure (or mass flow) of the oxidantduring a predetermined time period.

The library may, for example, indicate apparatus according to FIG. 2,tetracarbonyl nickel Ni(CO)₄ as the metal-containing reactant precursor203, O₂ as the oxidant, process chamber temperature, exposure times aswell as partial pressure (or mass flow) of oxidant.

In that case, the forming of a CEM switching device having apredetermined electrical impedance in its conductive state may compriseforming a CEM layer of a thin film of nickel oxide which is doped to apredetermined extent on the semiconductor wafer by atomic vapourdeposition employing cycles of the following operations.

First, the up-line valves for the purge gas are closed and the up-linevalves for the are opened. After a predetermined time period in whichthe semiconductor wafer is exposed to and reacts with tetracarbonylnickel Ni(CO)₄, the up-line valves for tetracarbonyl nickel Ni(CO)₄ areclosed and the up-line valves for the purge gas are reopened.

After a predetermined time period, the up-line valves for the purge gasare closed and the up-line valves for O₂ are opened. After apredetermined time period in which the semiconductor wafer is exposed toand reacts with O₂ at the specified partial pressure and temperature,the up-line valves for o₂ are closed and the up-line valves for thepurge gas are reopened.

The number of cycles of these operations is selected to provide a thinfilm layer on the semiconductor wafer of a desired thickness on thesemiconductor wafer.

The time period during which the semiconductor wafer or thin film layeris exposed to O₂ is selected so that the oxygen gas flow during thatperiod is insufficient for complete reaction of the reactive sites onthe semiconductor wafer or thin film layer with oxygen.

The gas flows during this time period can be easily adjusted by the massflow controller so that they are different. The adjustment enables afine tuning in the amount of dopant in the thin film layer.

The final nickel oxide thin film is obtained by an annealing carried outin the process chamber 202 during a predetermined time period in whichpurging with nitrogen is maintained. The temperature of the processchamber 202 and/or the pressure therein may be maintained or adjusted toa selected value or values during this predetermined time period.

FIG. 4 shows the gas flows in the apparatus during the formation of twoCEM layers having a differing extent of doping by an atomic vapourdeposition as described above.

The gas flows exhibit pulse profiles showing the relative amounts ofmetal-containing reactant precursor, oxidants and purge gas during thepredetermined periods for formation of the CEM layers.

As may be seen, the relative amount of oxidant for the atomic layerdeposition of the CEM layer with a higher extent of doping by carbonylis greater than the relative amount of oxidant for the atomic layerdeposition of the CEM layer with a higher extent of doping by carbonyl.The predetermined time period of exposure to the semiconductor wafer isthe same for each of the reactant precursors and the two oxidants.

1. A method for the manufacture of a correlated electron material (CEM)device comprising forming a CEM thin film of a correlated electronmaterial having a predetermined electrical impedance when the CEM deviceis in its relatively conductive (low impedance) state, wherein theforming of the CEM thin film comprises forming a d- or f-block metal ormetal compound by a physical or chemical vapour deposition with apredetermined amount of a dopant comprising a back-donating ligand forthe metal to impart the predetermined electrical impedance, and whereinthe predetermined electrical impedance is selected to match aconductivity of the CEM device with a transductance of one or morein-series field effect transistors.
 2. The method according to claim 1,comprising forming the CEM thin film by a chemical vapour deposition,such as an atomic layer deposition.
 3. (canceled)
 4. The methodaccording to claim 1, comprising forming the CEM thin film with anelectrical conductivity between 10³ S/m and 10¹⁰ S/m.
 5. The methodaccording to claim 1, comprising forming the CEM thin film withcross-sectional area between 25 nm² and 500 nm².
 6. The method accordingto claim 1, wherein the back-donating ligand for the metal is selectedfrom the group of back-donating ligands consisting of carbonyl,nitrosyl, isocyanide, dioxygen, dihydrogen, alkene, alkyne orphosphinyl.
 7. The method according to claim 1, wherein theback-donating ligand for the metal comprises one or more molecules offormula C_(a)H_(b)N_(d)O_(f) (in which a≥1, and b, d and f≥0) such as:carbonyl (CO), cyano (CN⁻), ethylenediamine (C₂H₈N₂),1,10-phenanthroline (C₁₂H₈N₂), bipyridine (C₁₀H₈N₂), pyridine (C₅H₅N),acetonitrile (CH₃CN) and cyanosulfanides such as thiocyanate (NCS⁻). 8.The method according to claim 1, wherein the d-block metal or metalcompound comprises nickel, iron, cobalt, ruthenium, rhodium, osmium oriridium.
 9. The method according to claim 1, wherein the forming of theCEM thin film comprises forming nickel oxide with a predetermined amountof a dopant comprising a back-donating ligand for nickel.
 10. Acorrelated electron material (CEM) device comprising a CEM thin film ofa correlated electron material having a predetermined electricalimpedance when the CEM device is in its conductive state wherein the CEMthin film comprises a d- or f-block metal or metal compound doped with apredetermined amount of a back-donating ligand for the metal to impartthe predetermined electrical impedance, and wherein the CEM thin filmhas a predetermined electrical impedance selected to match aconductivity of the CEM device to a transductance of one or morein-series field effect transistors.
 11. (canceled)
 12. The deviceaccording to claim 10, wherein the CEM thin film has an electricalconductivity between 10³ S/m and 10¹⁰ S/m.
 13. The device according toclaim 10, wherein the CEM thin film has a cross-sectional area between25 nm² and 500 nm².
 14. The device according to claim 10, wherein theCEM thin film comprises a transition metal oxide selected from the groupconsisting of oxides of nickel, iron, cobalt, ruthenium, osmium, iridiumand mixtures thereof.
 15. The device according to claim 10, having aleakage current density (viz., a parasitic MIM diode current density) inits insulative state less than or equal to 500 A/cm².
 16. A method fortuning electrical conductivity (or impedance) of a correlated electronmaterial (CEM) device in its relatively conductive (low impedance) stateto the transductance of one or more in-series field effect transistorsin an electrical circuit, which method comprises steps of forming afirst thin film of a correlated electron material by a physical orchemical vapour deposition of a d- or f-block metal or metal compoundwhich is doped by a first amount of back-donating ligand for the metal;comparing the electrical conductivity of the first thin film in itsrelatively conductive state to a transductance of the one or morein-series field effect transistors; and repeating these steps by formingone or more other thin films of a correlated electron material eachhaving a different amount of back-donating ligand for the metal ascompared to the first thin film layer until the electrical conductivityof one of the other thin films in its relatively conductive statematches the conductivity of the device to the transductance of the oneor more in-series field effect transistors.
 17. The method according toclaim 16, wherein the forming of the first and the one or more otherthin films of correlated electron material comprises atomic layerdeposition.